1. Field of the Invention
The invention relates to acousto-electric devices which may be used to perform memory, correlation or convolution operations.
2. Description of the Prior Art
One approach to convolution and correlation of an input signal with a reference signal has been to transduce the signals onto a piezoelectric surface and integrate the electric fields of the resulting acoustic waves in an adjacent semiconductor. The acoustic wave of the reference signal propagates from a reference transducer along the piezoelectric surface giving rise to normal and tangential electric fields. At the same time, the acoustic wave of the input signal is launched from an input transducer in the opposite direction to the reference wave. The normal component of the electric fields produced by the waves accumulate electrons in the adjacent semiconductor in a non-linear manner to produce a current proportional to the product of the local amplitude of the two signals. A summing electrode on the back surface of the semiconductor integrates the current over the interaction interval to produce an output proportional to the convolution of the two surface wave signals. Correlation of the input signal with the reference signal may be accomplished in substantially the same manner provided the input signal is first time inversed. The disadvantages of this approach are: that the reference wave must be periodically introduced; that the output is compressed in time by a factor of two; and that, for correlation, the input wave must be time inversed. As recognized in a technical paper entitled "New Adaptive-Signal-Processing Concept" by E. Stern and R. C. Williamson in Volume 10, Number 5 of Electronic Letters, these disadvantages can be eliminated by storing the reference wave in a suitable memory device.
In the prior art, memory devices have been proposed which store a reference signal in surface states of a semiconductor. Publications which have discussed this technique include: "Surface State Memory in Surface Acoustoelectric Correlator" by A. Bers and J. H. Cafarella in Applied Physics Letters, Volume 25, Number 3; and "Storage of Acoustic Signals in Surface States in Silicon" by H. Hayakawa and G. S. Kino in Applied Physics Letters, Volume 25, Number 4. The reference wave is transduced onto a piezoelectric surface where it forms an acoustic wave which propagates along the piezoelectric surface. When the acoustic wave is opposite the semiconductor, the semiconductor is excited by a short "write" pulse. This causes the normal electric field components of the piezoelectric surface wave to attract electrons to the adjacent semiconductor surface where they are captured by surface states. The captured electrons create a space-charge pattern at the semiconductor surface which is a spatial replica of the reference wave. The reference signal is retrieved by convolving a carrier wave and a read pulse which have been transduced onto the piezoelectric material. The reference signal is the amplitude modulation of the carrier wave which is provided at the output of the summing electrode attached to the semiconductor. Using this acousto-electric device, a real-time convolution of the stored reference signal with an input signal is obtained at the output of the summing electrode by transducing the input wave onto the piezoelectric surface so that it propagates in the same direction as the reference wave. A real-time correlation of the reference signal and an input signal is obtained, without time inversion of either signal, by transducing the input wave onto the piezoelectric surface so that it propagates in the opposite direction from the reference wave. Real-time convolution and correlation of the input and reference waves may be accomplished in this manner as long as the stored reference wave persists.
The reference signal storage time of these prior art devices is determined by the thermal emission of the trapped electrons from the surface states. When a large time-bandwidth product is required as, for example, in real-time matched filter radar applications, the ratio of storage time to writing time should be 10.sup.6. Storage of the reference signal in surface states, however, provides a storage to writing-time ratio of only 10.sup.4. For these large time-bandwith applications there is, therefore, a need for a device which would provide longer reference signal storage times. In relation to this problem, a publication entitled "A Schottky-Diode Acoustic Memory and Correlator" by K. A. Ingebrigtsen, R. A. Cohen and R. W. Mountain appeared on June 1, 1975 in Applied Physics Letters Volume 26, Number 11. A device according to such publication has a relatively low barrier, high leakage current, and short storage time.